2. I'm going downtown to see a man about a horse.
Many people will know what it means when you say "I have to go see a man about a horse." When I was in Mongolia which is truly a horse culture, my interpreter said, "I have to go see a horse".
We both laughed quite a bit that the same experession was common in both our cultures. I've often wondered why the American version has added "the man"?
Cause "seeing a horse" may have meant you were going to
rustle one which in certain parts of the country may have been
a capital offense. So it was better to say you were going
to see some one (certainly not a woman) about a horse. Cause
if you were going to see a woman about a horse, you may have
meant you were going to "horse around" with the woman!
Anyway, it's just my poor etymological guess.
I would imagine it's because when you say "I have to go see a man about a horse" it sounds like you have some really important business to take care of. That's the part that makes it funny. Maybe you're going to go buy or sell a horse. Saying "I have to go see a horse" doesn't sound all that impressive.